Given a table explaining the syntax of the NAL unit'a. How to interpret it?

Question

C++ / C#

Good afternoon.
I delve into the h.265\HEVC standard, there is a table that should explain the syntax of NAL units. But it only raises questions.

The second table, dedicated to the header of the NAL unit, is clear to me. It explains the structure of the unit header, that it consists of two bytes, and how many bits are allocated for which part. Although I did not understand why the parentheses after nal_unit_header. For, as I understand it, it is enough to create a structure \ class that includes components.
But here is the first table completely confused me. The Nal unit is described as a function that takes NumBytesInNalUnit - the number of bytes in the Nal unit. That's just the number of bytes in the Nal unit is not specified anywhere.
The standard itself describes the meaning of individual parts of the code, but this did not give me anything.
next_bits(n) - is it figuratively given here? Because I did not find such a function. As well as individual elements, like rbsp_byte.
And what is this function? How to interpret it? How to find out the size of NAL units?
Just in case, here is the standard. Page 51.
(I added a link, but it does not display)

Answer 1

Standard .
next_bits( n ) is described just above the tables:
next_bits( n ) - Returns the next n bits in the bitstream.
nal_unit() and nal_unit_header() are not functions - this is a description of structures, when the structure itself has a floating size depending on the parameters.
nal_unit_header() - does not depend on anything and its size is fixed.
nal_unit() depends on NumBytesInNalUnit, this parameter must be known in advance. For example, it may have been read earlier from the same bitstream or specified in a different way.
The loop calculates the size of the rbsp_byte array.
In section 7.4 tables are described in words.